Display damaged. Unable to use external monitor to access iMac

Ouch. What I feared might be the case, does that means that I can't use external monitor without the GPU?

@michaeleh - That changes things a bit! You'll Still need a new display panel as yours is broken. But you'll then need a controller board to replace the logic board if you want to convert it to an external monitor.

Basically, you are just using the shell, the LCD panel and the power supply everything else is taken out.

I'm still not sure given the costs is worth it.

@danj It depends on whither the cost of repair (if I do it myself) is less than half the cost of buying the same or new mac then it *might* be worth the trouble _IF_ there isn't any other underlying problems. That is main reason to try to get into the system somehow to see what else is wrong. If a bad GPU is the only reason why external monitor won't work then it's an obvious fix. Else replacing the GPU and LCD would be pointless. If there is some other component or software that is preventing external monitor, it gets dicey. Do I continue or sell it as a parts machine. I'm trying pinpoint the root problem then rest is just fix it up to working computer.

@michaeleh - Hold on here! Now you've changed things again!

A working iMac is replacing the GPU. Converting the display into an external display replaces the logic board with a 3rd party controller board (A for B).

The GPU feeds the video to both internally and external displays. So it's clear the GPU has failed.

You stated you get a boot up chime so the logic board its self is working!

@danj Thanks for making things clear. I'm not interested gutting an iMac into a monitor. It wouldn't really make sense when I can go out and buy a cheap 21" monitor anywhere. I'm just trying to rule out anything else before proceeding.

In another thread, they mention the diagnostic LEDs and trying to peek through bottom grilling to the right of ram slots, it showing all 4 LEDS lit. 3 is display is talking to the board. The 4th is lit which some posts saying this temperature error. Another saying the video is okay.

Boots up with a black screen

Forgive me if it seems like I trying to get out changing the GPU. I'm trying to diagnose any other issues that also might also be not obvious. Usually, people would change the LCD then still get black screen then change the GPU only to find something else wrong. I'm trying to get to the root cause so I can decide IF I'm going to fix it at all.